The invention relates to a method for producing a photovoltaic solar cell.
A photovoltaic solar cell represents a planar semiconductor element, in which via incident electromagnetic radiation a generation of electron-hole pairs is yielded, and a separation of charge carrier occurs at least at one pn-transfer so that an electric potential difference develops between at least two electric contacts of the solar cell and via an external circuit connected to these contacts electric power can be tapped from the solar cell.
Typical solar cells comprise a silicon semiconductor substrate of a base doping, with at least one emitter section of an emitter doping being embodied in said semiconductor substrate, which is opposite in reference to the base doping, so that the above-mentioned pn-transfer forms between the base section and the emitter section. The base section and the emitter section are each contacted by at least one metallic contact structure to collect and conduct the charge carriers.
In order to yield high effectiveness here an optimization must occur with regards to several loss mechanisms:
A band structure is given inside a crystalline semiconductor, such as silicon. It is disturbed at the surface of the semiconductor, because here e.g., photo-generated charge carriers can be combined more easily. This loss mechanism particularly reduces the no-load voltage, but also the short-circuit current density of solar cells, thus lowering their effectiveness. By applying a dielectric layer at sections of a solar cell, which cannot be contacted later on, such recombination loss can be reduced. This is called an improved passivation of the surface. The improved passivation may be obtained in various fashions:
On the one hand, by positive or negative charges appearing at the surface, due to the Coulomb repulsion of charged particles, a type of charge carriers, thus negatively charged electrons or positively charged defect electrons, also called holes, may be eliminated from the surface. Due to the fact that the recombination depends on the product of the densities of charge carriers here, by a suitable selection of charge carriers, i.e. positive or negative charges, and the amount of applied charge carriers the recombination rate can be reduced. On the one hand, the recombination of the surface defects occurs similar to open bonds. Any reduction of this so-called interface density by chemical bonding with an applied dielectric layer is important to allow higher effectiveness.
In general, any passivation is based on a combination of these two effects. Another possibility to reduce the loss by recombination at the surface of a semiconductor is to introduce a band bending in the semiconductor by introducing a doping atom by way of diffusion. For this purpose, generally atoms are used from the third or fifth main groups. In case of silicon being the semiconductor material, primarily phosphorus, boron, and aluminum are used, here. They act similar to applied charges and shield the surface from one type of charge carriers.
At the front of a semiconductor disk, such as a silicon wafer, presently in most wafer-based solar cell—production lines, a dielectric layer is applied, e.g., silicon nitride. This may be achieved e.g., by plasma-enhanced chemical vapor deposition (PECVD) or by cathode evaporation. Here, the dielectric layer serves for passivation of the surface, however it also forms an optically particular beneficial adjustment to the solar spectrum in order to reduce reflection loss by the various diffraction indices of silicon and the ambient material, e.g., air or polymer-based encapsulating films or modular glass.
Presently, no dielectric layers are used at the back of silicon solar cells in the industrial production because contacting generally occurs over the entire surface using printed and/or e.g., dried pastes and a subsequent tempering step to form contacts. Any dielectric passivation with an only locally contacted back allows a considerable increase of effectiveness of the solar cell, however it is technologically more expensive and thus, due to the higher costs, it is not used in present industrial production.
Furthermore, in order to yield high effectiveness optimization is required with regards to the following loss mechanisms:
For example, high doping is advantageous in order to form a low electric contact resistance between metal layers and contacted semiconductor sections. On the other hand, higher doping generally leads to a higher recombination of electron hole—pairs within the semiconductor substrate.
Accordingly, both in the emitter section as well as in the base section it is known to form selective doping structures.
For example, the embodiment of selective emitter structures is known, in which here on the front of the semiconductor substrate, facing the light, an emitter is embodied in a planar fashion with a first doping profile, and only in the sections in which contacting the emitter shall be embodied by a metallic emitter—contract structure applied on the front of the semiconductor structure, a second emitter profile is embodied with a lateral conductivity and surface doping which is higher in reference to the first profile. Such a selective emitter ensures that in a lateral conduction of charge carriers a reduction of recombination is yielded within the emitter section of the first emitter profile and on the other side, due to the higher doping, particularly the higher surface concentration of the second doping profile, a lower contact resistance is yielded in reference to the metallic contact structure.
Ultimately, here at the surfaces of a semiconductor substrate for the production of a solar cell typically both p-doped and n-doped sections are provided as well as the high-doped and low-doped sections of the respective doping type, i.e. sections with high and in reference thereto low concentration of surface doping.